1. Technical Field
The present invention relates to an integrated circuit, a vibrating device, an electronic apparatus, a moving object, and a mode switching method for the integrated circuit.
2. Related Art
Temperature Compensated X'tal Oscillators (TCXOs) are widely used for an apparatus and systems requiring high-accuracy timing signals of terminals and base stations of cellular phones, GPS (Global Positioning System) receivers, etc. because high frequency stability is obtained by cancelling out of deviation (frequency deviation) from a desired frequency (nominal frequency) of an oscillation frequency of a crystal oscillator in a predetermined temperature range.
Generally, for the TCXO, frequency adjustment with respect to each temperature is performed before shipment for temperature compensation of the oscillation frequency. In the adjustment, if the temperature change of the crystal vibrator contained in the TCXO and the detected temperature change of the temperature sensor within the IC for TCXO are relatively equal, high-accuracy temperature compensation can be performed. However, the temperature sensor is often mounted on the IC for TCXO and likely to be affected by heat generation due to consumed power of the IC for TCXO and the heat generation change with the consumed power change of the IC for TCXO affects only the temperature sensor, and thereby, the temperature change of the crystal vibrator and the detection temperature change of the temperature sensor may be unequal and, as a result, high-accuracy temperature compensation is hindered. For the problem, the IC for TCXO in related art has avoided any significant internal state change that causes heat generation change even in a serial I/F mode used for serial communication with an built-in memory or the like.
On the other hand, because of recent price reduction of TCXO, there has been an urgent need to reduce the area and the cost of the IC for TCXO contained in the TCXO, and an IC with the smaller area formed in a microfabrication process has been necessary. Further, the IC for TCXO often has the smaller number of pins for reduction in area and cost, it is hard to provide any dedicated pin for switching to the serial I/F mode used for serial communication with an built-in memory or the like, and thus, the mode switching may be performed by application of a high voltage to a power supply terminal (Patent Document 1 (U.S. Pat. No. 5,724,009)).
However, when a high voltage is applied to the power supply terminal of the IC for TCXO manufactured in the microfabrication process, as a result, the high voltage is applied to a transistor within the IC and stress on the transistor is not negligible, and thereby, characteristic degradation including HCI (Hot Carrier Injection), NBTI (Negative Bias Temperature Instability), TDDB (Time Dependent Dielectric Breakdown) is problematic. For example, NBTI is a phenomenon that, when the state in which there is a potential difference (Negative Bias) between the gate and the substrate of a transistor is held in a long period at a high temperature, the transistor characteristics shift. For example, in the case where an output buffer circuit of an oscillation circuit is formed as shown in FIG. 15, when a high voltage is applied to the power supply terminal (VDD terminal), a voltage difference between the source (S) and the gate (G) of a PMOS transistor within the dotted circle becomes larger, and stress causing NBTI is applied to the transistor. The problem of characteristic degradation hinders the cost reduction by employment of the microfabrication process for the IC for TCXO and makes it difficult to ensure reliability of the device.